The present disclosure relates to a nozzle for a dissolved air flotation system, and more particularly, to a nozzle for a dissolved air flotation system which can effectively generate microbubbles even at low pressure and form the microbubbles in a more uniform size.
A Dissolved Air Flotation (DAF) method has been used as a water treatment method in water treatment equipment. A dissolved air flotation system using the DAF method has been applied to the pretreatment process of water treatment and seawater desalination equipment.
In the dissolved air flotation system, low-density suspended solids and floating matter, such as algae and organic compounds contained in inflow water are condensed by putting a coagulant, an aid coagulant, pH control chemicals and others in the inflow water. Microbubbles are generated that combine with the suspended solids and floating matter to thereby remove the suspended solids and floating matter.
The dissolved air flotation system includes a microbubble generator, a floating tank, and a scum remover. Dissolved gas flotation pressure reduction nozzles are shown in FIG. 1.
Water treatment using the dissolved air flotation system includes four steps, namely, condensation and floc formation of particles, generation of microbubbles, collision and adhesion between the microbubbles and the flocs in a mixing area, and a rise of the microbubble-floc mass in a flotation tank. Here, in order to generate microbubbles, a pump, an air injection compressor, and an air/circulating water contact tank (saturation tank) are needed. When air is dissolved and saturated in treated water and the pressure lowers rapidly, air dissolved in circulating water is discharged as microbubbles. The microbubbles adhere to the flocs, and then, agglutinates of the flocs and the microbubbles float to the surface of water in the flotation tank. In order to enhance efficiency of the dissolved air flotation system, microbubbles of a proper size must be generated, and in this instance, the nozzle is important.
Bubbles are adhered to the agglutinates, which float to the surface of the treated water. A high density of bubbles per gas volume can be generated when the bubbles reduce in size. In the water treatment process, a contact time between bubbles and flocs is increased when the flocs are distributed thinly and there are lots of microbubbles. The size of bubbles and the flotation speed of bubbles are inversely proportional. The size of bubbles generated in the dissolved air flotation system is 10 μm to 120 μm, and the average size is about 40 μm to 50 μm.
In order to generate and spray bubbles, a nozzle is used. To effectively spray the air, pressure must be reduced rapidly and a high turbulent state is needed. The speed of returned water whose pressure is reduced must be low enough not to break the flocs. In order to stir the returned water and the flocs well, an appropriate turbulence must be formed to promote contact between the bubbles and the flocs.
FIG. 1 illustrates examples of nozzles for generating microbubbles. FIG. 1(a) illustrates a WRC nozzle, FIG. 1(b) illustrates an NIWR nozzle, FIG. 1(c) illustrates a DWL nozzle, and FIG. 1(d) illustrates a RICTOR nozzle.
Each of the nozzles has the following characteristics. The NIWR nozzle and the WRC nozzle respectively have collision surfaces on a flow boundary. The DWL nozzle, the RICTOR nozzle and the NIWR nozzle change the direction of a flow path. The RICTOR nozzle and the DWL nozzle respectively have inclined surfaces at an end of the flow path.
In detail, the WRC nozzle was originally developed by Water Research Center and includes a collision plate, changes the direction of the fluid, and causes decompression through orifices with different diameters. In order to induce a momentary pressure drop in the WRC nozzle, a thin plate is perforated to form a hole, and a flow jet sprayed through the hole collides with a flat plate mounted at a slipstream side. The nozzle has a simple structure because it uses collision energy of the fluid, but has several disadvantages in that it exhibits severe performance changes depending on the pressure of the supplied fluid and in that it operates only at a relatively high pressure.
In the NIWR nozzle, a flow direction of the fluid is changed at an angle of 180 degrees because its fluid supply direction is opposed to its spray direction. A collision with the wall surface occurs where the flow direction is changed. The NIWR nozzle has several disadvantages in that it is complicated in structure and difficult to manufacture and in that it is difficult to maintain and repair when the hole is obstructed by foreign matter.
The DWL nozzle changes a fluid supply direction and a fluid spray direction at an angle of 180 degrees like the NIWR nozzle, and controls a flow rate by changing an area of a path using a stem at the middle part of the nozzle. Additionally, the DWL nozzle is designed to change a cross-sectional area according to flow paths so as to drop pressure. The nozzle is difficult to manufacture and maintain because its structure is complicated.
In the RICTOR nozzle, fluid is introduced into a conical diffusion tube through one orifice in a vertical direction. The fluid collides with the wall surface, and then, is discharged out. The RICTOR nozzle is relatively simple in structure, but is difficult to manufacture.